Intermediate frequency furnace smelting process of 316Ti material

Abstract

This invention relates to a medium-frequency furnace melting process for 316Ti material. This innovative process effectively solves the problems of high titanium loss rate and difficult inclusion control through the synergistic effects of full-process argon protection, pre-baking of ferrotitanium, two-stage addition of a specific composite refining agent, and strong stirring during final treatment. Compared with traditional methods, this invention can stably increase the titanium yield to over 92% and significantly improve the morphology and distribution of inclusions in the material, ultimately greatly enhancing the corrosion resistance and fatigue strength of 316Ti material. This process has strong adaptability, low overall cost, and is particularly suitable for the industrial production of high-quality special stainless steels.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel smelting technology, and relates to a medium-frequency furnace smelting process for 316Ti material. Background Technology

[0002] 316Ti is an austenitic stainless steel made by adding titanium to 316 stainless steel. The addition of titanium preferentially combines with carbon to form titanium carbide, avoiding intergranular corrosion caused by the precipitation of carbon-chromium compounds at grain boundaries. This significantly improves the material's stability and mechanical properties in high-temperature and corrosive environments. It not only possesses the excellent seawater and acid / alkali resistance of 316 stainless steel, but also maintains its structural strength at medium-high temperatures of 400-900℃. Therefore, it is widely used in petrochemical, marine engineering, nuclear power equipment, and other fields with stringent requirements for corrosion resistance and high-temperature resistance, making it an indispensable key structural material in high-end equipment manufacturing.

[0003] Currently, the industrial production of 316Ti materials mainly relies on electric arc furnaces or medium-frequency furnaces for smelting. Among these, medium-frequency furnaces have become the mainstream choice for small and medium-sized production enterprises due to their high heating efficiency, precise temperature control, and relatively low energy consumption. The smelting process typically includes core steps such as raw material melting, deoxidation and slag formation, alloying (adding ferrotitanium), and final refining. The core objective is to ensure the uniform distribution and effective retention of titanium elements by controlling temperature, gas, and the addition of refining agents, while removing harmful impurities such as sulfur, phosphorus, and oxygen from the molten steel, ultimately obtaining steel ingots or molten steel with qualified composition and uniform structure.

[0004] The existing process for smelting 316Ti material in medium-frequency furnaces has significant shortcomings: First, the titanium loss rate is high. If ferrotitanium is not fully baked or directly exposed to high-temperature furnace gas when added, it easily reacts with oxygen in the air to form titanium oxide. Moreover, traditional addition methods often cause ferrotitanium to float on the surface of molten steel, resulting in incomplete dissolution. Second, the refining effect is poor. Most processes use a single-stage addition of refining agents, making it difficult to simultaneously achieve efficient pre-deoxidation and titanium oxide spheroidization. Residual oxide inclusions in the molten steel will reduce the mechanical properties and corrosion resistance of the material. Third, the protective gas control is imperfect. Some processes only introduce inert gas at the top of the furnace, leaving air at the bottom, which can lead to secondary oxidation of the molten steel during smelting, affecting the stability of the finished product quality. These problems all limit the full realization of the performance of 316Ti material and the improvement of production efficiency. Summary of the Invention

[0005] The purpose of this invention is to provide a medium-frequency furnace melting process for 316Ti material, which features high titanium yield, precise composition control, good inclusion purification effect, and strong process stability. It aims to solve the technical problems of easy oxidation and burning loss of titanium elements and deterioration of titanium compound inclusion morphology when using a medium-frequency furnace for melting.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A medium-frequency furnace melting process for 316Ti material includes the following steps:

[0008] (a) Pre-deoxidation and slag formation: Under argon protection, the temperature of molten steel is controlled at 1620±5℃, and the first part of Ca-Si-Al-Mg based composite refining agent is added to the molten pool for pre-deoxidation to form a liquid slag layer, and the liquid slag layer is removed from the molten steel.

[0009] (b) Titanium alloying: Baked ferrotitanium is added to the molten steel treated in step (a) for alloying;

[0010] (c) Final treatment: Under argon protection, after the titanium iron treated in step (b) has completely dissolved, add the remaining second part of Ca-Si-Al-Mg based composite refining agent to the molten pool and stir evenly.

[0011] It should be noted that the process in this application is based on a medium-frequency induction furnace. Its core lies in creating and maintaining a low oxygen partial pressure melting environment by combining full-process argon protection with the addition of specific refining agents in stages, and optimizing the addition and processing of titanium alloying.

[0012] As a preferred embodiment of the present invention, the chemical composition of the Ca-Si-Al-Mg based composite refining agent, by mass percentage, comprises: Ca 35-45%, Si 20-30%, Al 15-20%, Mg 10-15%, with the remainder being unavoidable impurities. This specific ratio of refining agent possesses strong deoxidation, deep desulfurization, and excellent slag-forming properties, creating an optimal thermodynamic environment for the addition of titanium.

[0013] As a preferred embodiment of the present invention, the mass of the first part of the Ca-Si-Al-Mg based composite refining agent accounts for 50-70% of the total mass of the refining agent.

[0014] As a preferred embodiment of the present invention, in step (b), the ferrotitanium is baked at 600-700°C and kept at that temperature for 30-60 minutes before being added; the amount of ferrotitanium added should be such that the target titanium content in the molten steel is 0.3-0.5%. This step can remove adsorbed water and crystal water from the surface of the ferrotitanium, preventing water vapor from entering the molten steel and causing secondary oxidation and hydrogen-induced porosity risks.

[0015] As a preferred technical solution of the present invention, in step (b), when adding ferrotitanium, it is wrapped with a high-temperature resistant material and sunk to the bottom of the molten pool for addition, while the furnace top is covered with heat-insulating material.

[0016] As a preferred technical solution of the present invention, the stirring in step (c) is achieved by electromagnetic stirring or inert gas blowing, and the stirring time is 2-5 minutes.

[0017] As a preferred embodiment of the present invention, the argon gas is introduced at the bottom and top of the furnace.

[0018] As a preferred embodiment of the present invention, step (a) further includes the following operations:

[0019] During the formation of the liquid slag layer, when the temperature reaches 1500-1550℃, fluorite and steel purification agent are added to the bottom of the furnace for preliminary slag formation.

[0020] The beneficial effects of this invention are:

[0021] (1) This invention uses "pre-deoxidation slag formation" as a starting point, and under the protection of argon gas throughout the process, it provides a highly reducing atmosphere for the molten steel, which greatly suppresses the oxidation loss when titanium is added. The refining agent with specific components is added in two stages. The first stage creates a safe zone for titanium alloying, and the second stage further purifies the molten steel and treats titanium oxides, thereby stabilizing the titanium yield at a high level and ensuring the precise control of the finished product composition, especially the key titanium content, and batch stability.

[0022] (2) The present invention uses a Ca-Si-Al-Mg based composite refining agent and combines it with a strong stirring process, and its comprehensive refining effect is far superior to that of ordinary refining agents. Ca and Mg vapors can effectively modify brittle inclusions such as clustered Al2O3 and TiO2, transforming them into low-melting-point spherical calcium-titanium composite inclusions, thereby significantly improving the fatigue resistance and corrosion resistance of the material.

[0023] (3) This invention innovates the process based on the ordinary medium frequency furnace platform. It does not rely on expensive external refining equipment such as AOD and VOD. High-quality 316Ti material can be produced by optimizing the internal process and material design. It is particularly suitable for small and medium-sized casting enterprises. While ensuring product quality, it greatly reduces equipment investment and production costs. By accurately controlling the titanium yield and using recycled materials, it greatly reduces the loss of high-value alloying elements and production costs. Detailed Implementation

[0024] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with embodiments, is provided below.

[0025] General conditions: All experiments described below were conducted in a 2-ton medium-frequency induction furnace with a magnesia lining thickness of 22% of the furnace cavity diameter. Argon gas protection was used throughout the process. Aeration was provided at the furnace bottom via permeable bricks at a flow rate of 12 L / min·ton, and a gas curtain was formed at the furnace top via a sealed cover at a flow rate of 18 L / min·ton. A slight positive pressure of 35 Pa was maintained inside the furnace. In the initial stage of smelting, 0.2% fluorite (CaF2) and 0.1% steel purification agent (CaO-SiO2-Al2O3 pre-melted slag) were added to the furnace bottom for preliminary slagging.

[0026] Example 1

[0027] Refining agent composition and distribution: A Ca-Si-Al-Mg based composite refining agent is used, with the following chemical composition by mass percentage: Ca 40%, Si 25%, Al 18%, Mg 12%, and the balance being unavoidable impurities. The total amount of refining agent added is 1.0% of the molten steel mass.

[0028] Step (a) Pre-deoxidation and slag formation: The temperature of the molten steel is raised to 1620°C, and the first part of the refining agent, which accounts for 60% of the total refining agent mass, is added to form a liquid slag layer.

[0029] Step (b) Titanium alloying: Titanium iron, which has been baked at 650℃ for 40 minutes, is wrapped in a 0.8mm thick low-carbon steel feeder and sunk to the bottom of the molten pool for addition. The amount added is calculated based on a titanium yield η=92%.

[0030] Step (c) Final treatment: After the ferrotitanium has completely dissolved, add the remaining 40% of the refining agent. Raise the temperature of the molten steel to 1635℃ and stir vigorously with an electromagnetic stirrer for 3 minutes.

[0031] Example 2

[0032] Refining agent composition and distribution: The refining agent consists of: Ca 45%, Si 22%, Al 17%, Mg 12%, with the balance being impurities. The total amount added is 1.0%.

[0033] Step (a): When the molten steel temperature is 1620℃, add the first part of the refining agent, which is 55% of the total refining agent mass.

[0034] Step (b): Add the ferrotitanium after baking at 600℃ for 45 minutes, and calculate the amount to be added based on η=93%.

[0035] Step (c): After adding the remaining refining agent, use argon gas injection (pressure 0.5 MPa, time 3 minutes, insertion depth 1 / 3) for strong stirring, and the temperature of the molten steel rises to 1635℃.

[0036] Example 3

[0037] Refining agent composition and distribution: The refining agent consists of: Ca 38%, Si 20%, Al 20%, Mg 15%, with the balance being impurities. The total amount added is 1.2%.

[0038] Step (a): When the molten steel temperature is 1625℃, add the first part of the refining agent, which is 65% of the total refining agent mass.

[0039] Step (b): Add the ferrotitanium after baking at 700℃ for 35 minutes, and calculate the amount to be added based on η=95%.

[0040] Step (c): After adding the remaining refining agent, use electromagnetic stirring for 4 minutes to raise the temperature of the molten steel to 1640℃.

[0041] Example 4

[0042] Refining agent composition and distribution: The refining agent consists of: Ca 30%, Si 25%, Al 15%, Mg 5%, with the balance being impurities. The total amount added is 0.9%.

[0043] Step (a): When the molten steel temperature is 1615℃, add the first part of the refining agent, which is 50% of the total refining agent mass.

[0044] Step (b): Add the ferrotitanium after baking at 680℃ for 50 minutes, and calculate the amount to be added based on η=91%.

[0045] Step (c): After adding the remaining refining agent, use argon gas injection (pressure 0.4 MPa, time 4 minutes, insertion depth 1 / 2) for strong stirring, and the temperature of the molten steel rises to 1630℃.

[0046] Example 5

[0047] Refining agent composition and distribution: The refining agent consists of: Ca 40%, Si 15%, Al 13%, Mg 7%, with the balance being impurities. The total amount added is 1.1%.

[0048] Step (a): When the molten steel temperature is 1620℃, add the first part of the refining agent, which accounts for 70% of the total refining agent mass.

[0049] Step (b): Add the ferrotitanium after baking at 620℃ for 60 minutes, and calculate the amount to be added based on η=94%.

[0050] Step (c): After adding the remaining refining agent, use electromagnetic stirring for 2.5 minutes to raise the temperature of the molten steel to 1638℃.

[0051] Comparative Example 1

[0052] Based on Example 1, steps (a) and (c) are not protected by argon gas throughout the process, while the other conditions are the same as in Example 1.

[0053] Comparative Example 2

[0054] Based on Example 1, the ferrotitanium was added directly without baking, and the rest remained the same as in Example 1.

[0055] Comparative Example 3

[0056] Based on Example 1, strong stirring is not performed in the final treatment stage of step (c), and the rest is consistent with Example 1.

[0057] Comparative Example 4

[0058] Based on Example 1, all refining agents are added at once in step (a), no refining agents are added in step (c), and the rest is consistent with Example 1.

[0059] Comparative Example 5, based on Example 1, uses Al for deoxygenation:

[0060] Step (a): Instead of using a compound refining agent, add 0.03% aluminum granules (Al) for diffusion deoxidation.

[0061] Step (c): No refining agent is added; only electromagnetic stirring is performed.

[0062] Everything else remains the same as in Example 1.

[0063] Performance testing:

[0064] Titanium yield: tested according to GB / T 11170-2008 standard;

[0065] Inclusions: Inclusions were analyzed according to ASTM E45-18 standard;

[0066] Corrosion resistance: Pitting equivalent tested according to ASTM G48 standard;

[0067] Toughness: Fatigue strength was tested according to ASTM A240 standard.

[0068]

[0069] The test results show that the medium-frequency furnace smelting process for 316Ti material provided by this invention, through the synergistic effect of core methods such as "initial slag formation with fluorite and purifying agent, full-process argon protection, pre-baking of titanium-iron, two-stage addition of Ca-Si-Al-Mg based composite refining agent with specific composition, and strong stirring in the final treatment", achieves high and stable titanium yield under different refining agent ratios. The titanium yield in the examples is stable at a high level of 92%-96%, with precise composition control; all inclusions achieve good to excellent spheroidization; the Ca and Mg elements in the composite refining agent effectively modify brittle inclusions such as high-melting-point Al2O3 and TiO2 into low-melting-point spherical calcium-titanium composite inclusions; the corrosion resistance (PREN value ≥ 37.9) and fatigue strength (≥ 315 MPa) are both superior to the comparative example.

[0070] Comparative Example 1: Without argon protection, air intrusion led to secondary oxidation of the molten steel, resulting in significant oxidation of titanium, a sharp drop in yield, and contamination of the molten steel by oxidation products. This worsened inclusion ratings and significantly reduced corrosion resistance and fatigue strength. Comparative Example 2: The ferrotitanium was not baked, allowing moisture from its surface to enter the molten steel. This not only introduced oxygen, reducing yield, but also introduced hydrogen, causing porosity. The presence of gaseous elements severely damaged the material's density and mechanical properties. Comparative Example 3: Without strong stirring, the refining agent added in the second stage could not fully contact and react with the dispersed TiO2 inclusions, leading to spheroidization failure. Although the titanium yield was acceptable, the product contained numerous unmodified angular inclusions, which became fatigue crack initiation points, greatly reducing fatigue strength. Comparative Example 4: The refining agent was added all at once and was consumed before the titanium was added. During titanium alloying, the newly generated TiO2 lacked sufficient Ca and Mg for treatment and could only exist in a fine and dispersed solid form. It could not be spheroidized or floated, resulting in a decrease in performance. Comparative Example 5 used traditional Al deoxidation. Although Al has a strong deoxidation ability and the yield and total oxygen content seem good, the generated high-hardness, high-melting-point, clustered Al2O3 is an extremely harmful inclusion that seriously deteriorates the toughness and fatigue performance of the steel. At the same time, Al2O3 inclusions are also the preferred sites for pitting corrosion.

[0071] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A medium-frequency furnace melting process for 316Ti material, characterized in that, Includes the following steps: (a) Under argon protection, the temperature of the molten steel is controlled at 1620±5℃, and a first part of Ca-Si-Al-Mg based composite refining agent is added to the molten pool for pre-deoxidation to form a liquid slag layer, and the liquid slag layer is removed from the molten steel. (b) Alloying by adding baked ferrotitanium to the molten steel treated in step (a); (c) Under argon protection, after the ferrotitanium treated in step (b) has completely dissolved, add the remaining second part of Ca-Si-Al-Mg based composite refining agent to the molten pool and stir evenly; The chemical composition of the Ca-Si-Al-Mg based composite refining agent, by mass percentage, includes: Ca 35-45%, Si 20-30%, Al 15-20%, Mg 10-15%, with the remainder being unavoidable impurities; the amount of the Ca-Si-Al-Mg based composite refining agent added is 0.9-1.2%. Step (a) The mass of the first part of the Ca-Si-Al-Mg based composite refining agent accounts for 50-70% of the total mass of the refining agent; In step (b), the amount of ferrotitanium added should be such that the target titanium content in the molten steel is 0.3-0.5%.

2. The medium-frequency furnace melting process for 316Ti material according to claim 1, characterized in that, In step (b), the ferrotitanium is baked at 600-700°C and kept at that temperature for 30-60 minutes before being added.

3. The medium-frequency furnace melting process for 316Ti material according to claim 1, characterized in that, In step (b), when adding ferrotitanium, it is wrapped with high-temperature resistant material and sunk to the bottom of the molten pool for addition, while the furnace top is covered with heat-insulating material.

4. The medium-frequency furnace melting process for 316Ti material according to claim 1, characterized in that, The stirring in step (c) is achieved by electromagnetic stirring or inert gas blowing, and the stirring time is 2-5 minutes.

5. The medium-frequency furnace melting process for 316Ti material according to claim 1, characterized in that, The argon gas is introduced at the bottom and top of the furnace.

6. The medium-frequency furnace melting process for 316Ti material according to claim 1, characterized in that, Step (a) also includes the following operations: During the formation of the liquid slag layer, when the temperature reaches 1500-1550℃, fluorite and steel purification agent are added to the bottom of the furnace for preliminary slag formation.

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